BUG/MEDIUM: cache: replace old object on store

BUG/MEDIUM: cache: replace old object on store

Currently the cache aborts a store operation if the object to store
already exists in the cache. This is used to avoid storing multiple
copies at the same time on concurrent accesses. It causes an issue
though, which is that existing unexpired objects cannot be updated.
This happens when any request criterion disables the retrieval from
the cache (eg: with max-age or any other cache-control condition).

For now, let's simply replace the previous existing entry by unlinking
it from the index. This could possibly be improved in the future if
needed.

This fix needs to be backported to 1.8.